Hopefully someone more experienced than me will chip in, but if you mean into an unheated coop, with winter coming on, they need to be pretty well fully grown, I think. Maybe 4 months old. But if you have an enclosed, draft free, heated coop, they could go younger, of course. Depending on how many chicks you have, and how much room in your brooder. What are your conditions? # of chicks? Amount of room in your brooder? What type accommodations did you plan to move them to? How cold are your winters?